Dental amalgams are a blend or admixture which consists essentially of silver, tin and copper with the balance being mercury, and is filled and set in cavities in the teeth in the mouth.
As stipulated in JIS T 6109, the known dental amalgam alloys enjoying wide use until now are generally composed of, on the weight basis, 65% or higher of silver, 29% or less of tin, 6% or less of copper, 3% or less of mercury and 2% or less of zinc. However, the alloys complying with that standard have been found to be unsatisfactory as regards their mechanical properties and corrosion resistance due to the fact that the (.gamma..sub.2) phase of Sn.sub.7-8 Hg crystallized out in the set amalgams is poor in strength and corrosion.
As the amalgam alloys in which no crystallization of that (.gamma..sub.2) phase takes place, the so-called high-copper type amalgam alloys have recently been introduced, including those referred to as the so-called dispersion-strengthened type. Such alloys are characterized by their increased copper content that exceeds the value provided by the Japanese Industrial Standards (JIS). This feature suppresses crystallization of the (.gamma..sub.2) phase and, instead, effect crystallization of the (.epsilon.) phase and (.eta.) phase of Cu.sub.3 Sn and Cu.sub.6 Sn.sub.5 which contribute to improvements in mechanical properties.
With the known amalgam alloys as mentioned above, however, no care is taken of changes in the compressive strength in the initial packing stage (hereinafter referred to as the initial compressive strength) and of the toxicity to cells of the mercury leaching out of amalgam slurries. That is to say, a time period of several hours is required until the packed amalgam reaches a strength capable of resisting to initial biting forces. During this period, the packed amalgam may possibly be broken by external forces such as biting forces. In addition, when amalgam filling come in contact with the oral fluids in the course of time during which the amalgam sets sufficiently, the mercury leaches out of the amalgam slurry, and may accumulate inside the body.